The invention relates to an improved continuous process for preparing polyoxymethylenes.
It is known that oxymethylene polymers can be prepared by continuous bulk polymerization of the monomers in the presence of cationic initiators. This polymerization is frequently carried out in kneaders or in extruders. The temperature profile here can be such that the resultant oxymethylene polymer is produced either in solid form (DE-A 1 161 421, DE-A 1 495 228, DE-A 1 720 358, DE-A 3 018 898), or else as a melt (DE-A 3 147 309). The work-up of the polymer produced in solid form is known, and for this see: DE-A 3147309, DE-A 3628561, EP-A 678535, EP-A 699965 and DE-A 4423617.
The polymer produced as a melt is further processed in a downstream devolatilizing and finishing reactor. For this, the melt is conveyed directly from the extruder to this reactor without undergoing any phase change between these stages of the process (DE-A 3 147 309). In this downstream reactor, the thermal degradation of the unstable chain ends of the polymer takes place. The removal of the unconverted monomers and of the decomposition products arising during the thermal degradation, in particular formaldehyde, takes place by evaporation, mostly in vented extruders.
Since the monomers used are generally not completely converted to the polymer, their vapor pressure has a foaming effect on the polymer melt as soon as the pressure on the melt is reduced to atmospheric pressure (see DE-A 3 147 309).
It is an object of the present invention, therefore, to provide an improved continuous process for preparing polyoxymethylenes which has the following advantages over the prior art:
The tendency to form a foam during devolatilization should be reduced, and the production of fines during pelletization minimized. The pellets should be compact. The throughputs in the finishing extruder should be increased and the energy requirement lowered.
We have found that this object is achieved by means of a continuous process for preparing polyoxymethylene homo- or copolymers by bulk polymerization of the monomers in the presence of cationic initiators, and also, if desired, in the presence of regulators, where during the polymerization both the monomers and the polymer are present in molten form and, if desired, the polymer is then deactivated, and the melt is discharged, cooled and pelletized, which comprises discharging, cooling and pelletizing the polymer at an elevated pressure and in the presence of a liquid.
The process may in principle be carried out on any screw machinery with high mixing effectiveness. Preferred devices are extruders, Buss kneaders and flow tubes with or without static mixing elements, preferably twin-screw extruders.
In a first step of the novel process, polyoxymethylene homo- or copolymer is melted in a preferably heated zone.
Polymers of this type are known to the skilled worker and are described in the literature.
These polymers very generally have at least 50 mol % of repeat xe2x80x94CH2Oxe2x80x94 units in their main polymer chain.
The homopolymers are generally prepared by polymerizing formaldehyde or trioxane, preferably in the presence of suitable catalysts.
For the purposes of the present invention, polyoxymethylene copolymers are preferred, particularly those which, besides the xe2x80x94CH2Oxe2x80x94 repeat units, also have up to 50 mol %, preferably from 0.1 to 20 mol %, in particular from 0.3 to 10 mol %, and very particularly preferably from 2 to 6 mol %, of repeat units 
where R1 to R4, independently of one another, are hydrogen, C1-C4-alkyl or halogen-substituted alkyl having from 1 to 4 carbon atoms, and R5 is xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, C1-C4-alkyl- or C1-C4-haloalkyl-substituted methylene or a corresponding oxymethylene group, and n is from 0 to 3. These groups may be advantageously introduced into the copolymers by ring-opening of cyclic ethers. Preferred cyclic ethers have the formula 
where R1 to R5 and n are as defined above. Mention may be made, merely as examples, of ethylene oxide, propylene 1,2-oxide, butylene 1,2-oxide, butylene 1,3-oxide, 1,3-dioxane, 1,3-dioxolane and 1,3-dioxepan as cyclic ethers, and also linear oligo- and polyformals, such as polydioxolane or polydioxepan, as comonomers.
Other suitable oxymethylene terpolymers are those prepared, for example, by reacting trioxane, one of the cyclic ethers described above and a third monomer, preferably bifunctional compounds of the formula 
where Z is a chemical bond, xe2x80x94Oxe2x80x94 or xe2x80x94OROxe2x80x94 (Rxe2x95x90C1-C8-alkylene or C2-C8-cycloalkylene).
Preferred monomers of this type are ethylene diglycide, diglycidyl ether and diethers made from glycidyl compounds and formaldehyde, dioxane or trioxane in a molar ratio of 2:1, and also diethers made from 2 mol of glycidyl compound and 1 mol of an aliphatic diol having from 2 to 8 carbon atoms, for example the diglycidyl ethers of ethylene glycol, 1,4-butanediol, 1,3-butanediol, 1,3-cyclobutanediol, 1,2-propanediol or 1,4-cyclohexanediol, to mention merely a few examples.
Processes for preparing the homo- and copolymers described above are known to the person skilled in the art and are described in the literature, and further details would therefore be superfluous here.
The preferred polyoxymethylene copolymers have melting points of at least 150xc2x0 C. and molecular weights (weight average) Mw of from 5000 to 200,000, preferably from 7000 to 150,000.
Particular preference is given to end-group-stabilized polyoxymethylene polymers which have Cxe2x80x94C bonds at the ends of the chains.
The molten polymer causes what is known as melt sealing in the polymerization which follows, so that volatile constituents remain in the extruder, for example. The abovementioned monomers or a mixture of these, together or in succession with cationic initiators, are fed into the polymer melt. The temperature of the reaction mixture during the feed is preferably from 62 to 114xc2x0 C., in particular from 70 to 90xc2x0 C.
The novel process is preferably employed for the homo- or copolymerization of trioxane. However, in principle any of the monomers described above, including tetroxane, for example, may be used as monomer.
The monomers, for example trioxane, are preferably fed molten and generally at from 60 to 120xc2x0 C. Since the polymerization is generally exothermic, it is merely necessary to melt the polymer by supplying energy at the start of the novel process. The heat of polymerization generated is then sufficient to melt the polyoxymethylene homo- or copolymer formed.
The molecular weights of the polymer may, if desired, be adjusted to the desired values by using the customary (trioxane) polymerization regulators. Possible regulators are acetals and, respectively, formals of monohydric alcohols, the alcohols themselves, and also the small amounts of water whose presence can generally never be completely avoided and which function as a chain transfer agent. The amounts used of the regulators are from 10 to 10,000 ppm, preferably from 100 to 1000 ppm.
The initiators used are the customary cationic (trioxane) polymerization initiators. Suitable initiators are protonic acids, such as fluorinated or chlorinated alkyl- or arylsulfonic acids, e.g. trifluoromethanesulfonic acid, or Lewis acids, such as tin tetrachloride, arsenic pentafluoride, phosphorus pentafluoride and boron trifluoride, or else their complex compounds and salt-type compounds, e.g. boron trifluoride etherates and triphenylmethyl hexafluorophosphate. The amounts used of the catalysts are from about 0.01 to 1000 ppm, preferably from 0.05 to 10 ppm. It is generally advisable to add the catalyst in dilute form, preferably at concentrations of from 0.005 to 5% by weight. Solvents which may be used for this purpose are inert compounds, such as aliphatic or cycloaliphatic hydrocarbons, halogenated aliphatic hydrocarbons, glycol ethers, etc.
Monomers, initiators and, if desired, regulators may be premixed in any desired manner, or else added separately from one another to the polymerization reactor. There may also be provision of comonomer feed points along the reactor.
According to the invention, the temperature and pressure in the polymerization zone are to be selected in such a way that monomers and polymer are molten. For example, trioxane melts at from about 60 to 65xc2x0 C., and its boiling point at atmospheric pressure is 115xc2x0 C. Since the polymerization temperatures are relatively high, the polymerization generally takes place at elevated pressure, preferably from 1.5 to 500 bar, in particular from 5 to 50 bar. Under the reaction conditions, trioxane is in equilibrium with from about 1.5 to 2% of formaldehyde, at least some of which is present as a gas in the closed system. The oxymethylene homopolymer has a melting point of about 176xc2x0 C., and if relatively large amounts of comonomers have been incorporated the melting point can be lowered to about 150xc2x0 C., and it can be still further reduced by unconverted trioxane. The temperature of the melt in the polymerization reactor should not exceed 300xc2x0 C., since at high temperatures of this nature oxymethylene polymers decompose. The particularly preferred temperature range is from 150 to 200xc2x0 C. The melt temperature of the monomer/polymer mixture is very difficult to measure precisely, but some indication is given by the external temperature of the polymerization reactor. It is important that the polymer is produced in molten form.
The residence time of the polymerization mixture in the polymerization zone is preferably from 0.1 to 20 min, in particular from 0.4 to 5 min. The polymerization is preferably conducted to a conversion of at least 30%, in particular more than 60%. Under favorable conditions it is also possible to achieve conversions of 80% and above.
It is preferable for the polymerization mixture to be deactivated immediately following the polymerization, without any phase change taking place.
The deactivation of the catalyst residues generally takes place by adding deactivators to the polymerization melt. Examples of suitable deactivators are ammonia, aliphatic or aromatic amines, alcohols, salts which react as bases, such as sodium carbonate or borax, or else water. The amounts of these added to the polymers are preferably up to 1% by weight.
The spatial separation of the deactivation reactor with respect to the polymerization reactor is such that the polymer/monomer melt can move without hindrance from one to the other but penetration of the deactivators into the polymerization reactor is reliably prevented. The separation is brought about by constrictions incorporated, which locally increase the flow rate of the melt, for example by melt flow restrictors if extruders are used. The design of the deactivation reactor is such that there is thorough mixing of the polymer/monomer melt within a short time. In the case of extruders this can be achieved in practice by incorporating particular kneading elements or back-mixing elements. The temperature in the deactivation reactor is preferably from 150 to 300xc2x0 C. It is also possible for stabilizers to be added straightaway to the melt in the deactivation reactor, preferably together with the deactivators.